UPS systems and methods using coordinated static switch and inverter operation for generator walk-in

ABSTRACT

An uninterruptible power supply (UPS) system includes an AC output, an inverter coupled to the AC output and configured to provide power at the AC output, and a switch configured to selectively couple a generator (e.g., an engine-generator set) to the AC output. The system further includes a control circuit configured to variably modulate the switch to gradually increase control power flow from the generator to the AC output while causing the inverter to concurrently provide power to the AC output. The switch may be a static switch that includes at least one silicon controlled rectifier (SCR), and the control circuit may be configured to control a conduction interval of the at least one SCR to control power flow from the generator to the AC output.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/283,744, filed May 21, 2014 entitled UPS SYSTEMS AND METHODSUSING COORDINATED STATIC SWITCH AND INVERTER OPERATION FOR GENERATORWALK-IN, the disclosure of which is hereby incorporated by reference inits entirety.

BACKGROUND

The inventive subject matter relates to power conversion apparatus andmethods and, more particularly, to uninterruptible power supply (UPS)apparatus and methods.

UPS systems are commonly used in installations such as data centers,medical centers and industrial facilities. UPS systems may be used insuch installations, for example, to provide backup power to maintainoperation in event of failure of the primary utility supply.

UPS systems may have any of a number of different types ofarchitectures. For example, AC UPS systems may have an on-line or doubleconversion architecture including a rectifier configured to be coupledto an AC power source and an inverter coupled to the rectifier by a DCbus and configured to provide AC power to a load. A battery or other DCsource may be coupled to the DC source, which may provide backup powerin the event of failure of the AC source. Standby AC UPS systems mayinclude an inverter that is configured to be coupled to a load by atransfer switch that switches a load between the inverter and an ACsource. AC UPS system may have other architectures, such as lineinteractive and delta conversion architectures.

Large data centers have proliferated with the advent of web services andcloud computing. Some newer large data centers occupy millions of squarefeet and house hundreds of thousands of servers. Typically powered bythe local grid, these centers may include backup power supply systemsincluding UPSs and diesel or gas powered backup engine-generator sets tosupport continued operation when utility power is lost.

In a double conversion UPS, rectifier and inverter controls may besignificantly decoupled because of the presence of energy storage on theDC link between the rectifier and inverter. The rectifier can be used toaddress source compatibility issues (e.g., voltage droop, harmonics,power factor, distortion, etc.), while the inverter can be used toprotect the load (e.g., voltage, distortion, load regulation, etc.).

When an engine-generator set is used in a data center or similarapplication, the load applied to the engine-generator set is oftengradually increased to avoid transient overloading of theengine-generator set and tripping its protection mechanisms. A doubleconversion UPS can be used to gradually increase loading of anengine-generator set (to “walk in” the engine-generator set) by usingthe battery and inverter to provide power to the load while therectifier gradually increases loading of the engine-generator set. Suchgenerator walk-in techniques may be unavailable, however, in UPS systemsthat do not have a double conversion architecture.

SUMMARY

Some embodiments of the inventive subject matter provide anuninterruptible power supply (UPS) system including an AC output, aninverter coupled to the AC output and configured to provide power at theAC output, and a switch configured to selectively couple a generator(e.g., an engine-generator set) to the AC output. The system furtherincludes a control circuit configured to variably modulate the switch togradually increase control power flow from the generator to the ACoutput while causing the inverter to concurrent provide power to the ACoutput.

In some embodiments, the switch includes a static switch, e.g., a staticswitch that includes at least one silicon controlled rectifier (SCR).The control circuit may be configured to control a conduction intervalof the static switch to control power flow from the generator to the ACoutput. In some embodiments, the control circuit may be configured tochange the conduction interval in discrete steps over respectivediscrete time intervals to control the power flow from the generator tothe AC output. In some embodiments, the control circuit may beconfigured to vary the conduction interval responsive to a signalindicating a performance margin of the generator.

In some embodiments, the control circuit may be configured to operatethe inverter to provide reactive power compensation as power transferfrom the generator to the AC output gradually increases. In furtherembodiments, the control circuit may be configured to operate theinverter to provide harmonic compensation as power transfer from thegenerator to the AC output gradually increases.

Further embodiments of the inventive subject matter provide a systemincluding a utility power source, an engine-generator set, a staticswitch configured to couple the engine-generator set to a load, and aninverter having an output configured to be coupled to the load. Thesystem further includes a control circuit operatively coupled to theengine-generator set, the static switch, and the inverter and configuredto detect a failure of the utility power source, to activate theengine-generator set and to variably modulate the static switch togradually increase control power flow from the engine-generator set tothe load while causing the inverter to concurrently provide power to theload. The static switch may include at least one SCR and the controlcircuit may be configured to control a firing angle of the at least oneSCR to control power flow from the generator and the load. For example,in some embodiments, the control circuit may be configured to change aconduction interval of the at least one SCR in discrete steps overrespective discrete time intervals to control the power flow from thegenerator to the AC output. In some embodiments, the control circuit maybe configured to vary a conduction interval of the SCR responsive to asignal indicating a performance margin of the generator. The controlcircuit may be further configured to operate the inverter to providereactive power compensation and/or harmonic compensation.

Methods according to some embodiments of the inventive subject matterinclude providing AC power to a load from an AC power source. A loss ofthe AC power source is detected, and the load is responsivelytransitioned to an inverter of a UPS. A generator is activatedresponsive to detected loss of the AC power source. A switch configuredto couple the generator to the load is variably modulated to graduallyincrease control power flow from the generator to the load while theinverter concurrently provides power to the load.

The switch may include a static switch including at least one SCR andvariably modulating the switch may comprise controlling a conductioninterval of the at least one SCR to gradually increase control powerflow between the generator and the AC output. Controlling the conductioninterval of the at least one SCR may include changing the conductioninterval in discrete steps over respective discrete time intervals. Insome embodiments, controlling the conduction interval of the at leastone SCR may include varying the conduction interval responsive to asignal indicating a performance margin of the generator. The invertermay also be used to provide reactive power compensation and/or harmoniccompensation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a UPS system according tosome embodiments.

FIG. 2 is a schematic diagram illustrating a UPS system coupled to autility source and an engine-generator set according to furtherembodiments.

FIG. 3 is a flowchart illustrating operations of a UPS system of FIG. 2according to further embodiments.

FIG. 4 is a schematic diagram illustrating a UPS system coupled to autility source and engine-generator set according to furtherembodiments.

FIG. 5 is a flowchart illustrating operations of a UPS system of FIG. 2according to further embodiments.

FIG. 6 is a schematic diagram illustrating a three-phase UPS systemaccording to some embodiments.

FIGS. 7-10 are flowcharts illustrating exemplary operations of UPSsystems according to further embodiments.

DETAILED DESCRIPTION

Specific exemplary embodiments of the inventive subject matter now willbe described with reference to the accompanying drawings. This inventivesubject matter may, however, be embodied in many different forms andshould not be construed as limited to the embodiments set forth herein;rather, these embodiments are provided so that this disclosure will bethorough and complete, and will fully convey the scope of the inventivesubject matter to those skilled in the art. In the drawings, likenumbers refer to like elements. It will be understood that when anelement is referred to as being “connected” or “coupled” to anotherelement, it can be directly connected or coupled to the other element orintervening elements may be present. As used herein the term “and/or”includes any and all combinations of one or more of the associatedlisted items.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the inventivesubject matter. As used herein, the singular forms “a”, “an” and “the”are intended to include the plural forms as well, unless expresslystated otherwise. It will be further understood that the terms“includes,” “comprises,” “including” and/or “comprising,” when used inthis specification, specify the presence of stated features, integers,steps, operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this inventive subject matterbelongs. It will be further understood that terms, such as those definedin commonly used dictionaries, should be interpreted as having a meaningthat is consistent with their meaning in the context of thespecification and the relevant art and will not be interpreted in anidealized or overly formal sense unless expressly so defined herein.

Some embodiments of the inventive subject matter arise from arealization that in applications such as data center uninterruptiblepower supply (UPS) systems, a UPS system having no regulator andinverter DC link to implement conventional generator walk-in, i.e., a“rectifierless” UPS, may operate an inverter and a static switch in acoordinated manner to provide generator walk-in. According to someembodiments, the generator and inverter may be coupled in parallel tothe load and generator walk-in may be achieved by using phase control(e.g., firing angle control) of a waveform derived from the generator inconjunction with phase/frequency control of the inverter, e.g., powertransfer from the generator is gradually increased as the inverterperforms complementary operations to maintain a desired input (voltageand/or current) to the load. As used herein, “gradual” increase in powertransfer from a generator to a load refers to a general trend toincrease power transfer to the load from the generator. It will beunderstood that a “gradual” increase in power transfer may includeperiods in which the power transfer remains constant and/or decreases(e.g., to preserve generator stability as described below) within ageneral trend of increasing power transfer. In some embodiments, such agradual increase in power transfer may be achieved by using a conductioninterval of the static switch to limit the amount of power drawn fromthe generator to prevent tripping of protective devices or other eventsthat may accompany an overly large step increase in generator load.Inverter phasing/frequency may be used to maintain the load as theconduction interval is increased. The inverter can also be used forpower factor correction and/or harmonic compensation.

FIG. 1 illustrates a system 100 according to some embodiments of theinventive subject matter. The system 100 includes a switch 110, which isconfigured to couple and decouple a generator 10 to and from a load 20.The system further includes an inverter 120 having an output coupled tothe load 20. A control circuit 130 is configured to variably modulatethe switch 110 to gradually increase power transfer from the generator10 to the load 20 while concurrently causing the inverter 120 tomaintain the load by providing additional power. For example, inembodiments described below, a conduction interval of the switch 110 maybe incrementally increased to gradually load the generator 10. Theinverter 120 may also be operated to control power factor and/orharmonics.

It will be appreciated that the system 100 may be implemented in anumber of different ways. For example, the switch 110, the inverter 120,and the control circuit 130 may be incorporated in one or more units(e.g., in a UPS unit) or may be distributed. The switch 110 may be asemiconductor switch, such as a switch utilizing silicon controlledrectifiers (SCRs), insulated gate bipolar transistors (IGBTs), powerMOSFETs or other power semiconductor devices. The inverter 120 mayemploy a half-bridge or other configuration. The control circuit 130 maybe implemented using any of a variety of different analog and/or digitalcircuits, including data processing devices, such as a microcontroller,and may be integrated with the inverter 120 and/or the switch 110 (e.g.,in an integrated UPS assembly) or distributed among multiple suchcomponents. The generator 10 may be engine-generator set, such as adiesel- or gas-powered engine-generator set.

FIG. 2 illustrates a system 200 according to further embodiments. Autility source 12 and an engine-generator set 14 are coupled to inputsof a transfer switch 210. An output of the transfer switch 210 iscoupled to a static switch 220, which has an output coupled to a load 20in parallel with an inverter 230. The inverter 230 is coupled to a DCpower source 240, which may include, for example, a battery and DC/DCconverter for interfacing the battery to the inverter 230. A controlcircuit 250 is operatively coupled to the inverter 230, the staticswitch 220, the transfer switch 210 and/or the engine-generator set 14to allow control and/or monitoring thereof.

Operations of the system 200 according to some embodiments of theinventive subject matter are illustrated in FIG. 3. The transfer switch210 is configured to selectively couple the utility source 12 and theengine-generator set 14 to the static switch 220 based on a status ofthe utility source 12. Under normal conditions, the load 20 is poweredfrom the utility source 12 (block 310). If the utility source 12 fails,the control circuit 250 may cause the static switch 220 to open, causethe inverter 230 to maintain the load 20 using power from the DC powersource 240, activate the engine-generator set 14, and cause the transferswitch 210 to couple the engine-generator set 14 to the input of thestatic switch 220 (blocks 320, 330). Activation of the engine-generatorset 14 and operation of the transfer switch 210 may be coordinated invarious ways, e.g., the transfer switch may 210 may be transitionedbefore and/or concurrent with activation of the engine-generator set 14or may be delayed until the engine-generator set 14 reaches apredetermined operating state after activation. Once theengine-generator set 14 is on line and its output available at theoutput of the transfer switch 210, the control circuit 250 may thenvariably modulate the static switch 220 (e.g., by varying a conductioninterval thereof) while operating the inverter 230 as described above togradually walk-in the engine-generator set 14 (block 340). During thisprocess, the inverter 230 may be used to maintain the load and providereactive power and/or harmonic compensation as described above. Once theengine-generator set 14 is fully able to carry the load 20, the inverter230 may be deactivated (blocks 350, 360). In some embodiments, however,the inverter 230 may remain active and be used to continue to providereactive power compensation, harmonic compensation and/or other powerconditioning. The inverter 230 may also be operated to facilitate powerflow from the engine-generator set 14 to the DC source 240 for batterycharging.

FIG. 4 illustrates a system 400 according to further embodiments with adifferent configuration from the system 200 of FIG. 2. A utility source12 and an engine-generator set 14 are coupled to inputs of respectivefirst and second static switches 410, 420 having outputs configured tobe coupled in common to a load 20, in parallel with an inverter 430. Theinverter 430 is coupled to a DC power source 440 (e.g., a battery andDC/DC converter). A control circuit 450 is operatively coupled to theinverter 430, the first and second static switches 410, 420 and/or theengine-generator set 14 to allow control and/or monitoring thereof.

Operations of the system 400 according to some embodiments of theinventive subject matter are illustrated in FIG. 5. Under normalconditions, the load 20 is powered from the utility source 12 (block510). If the utility source 12 fails, the control circuit 450 may causethe first static switch 410 to open, cause the inverter 430 to maintainthe load 20 using power from the DC power source 440, and activate theengine-generator set 14 (blocks 520, 530). Once the engine-generator set14 is activated, the control circuit 450 may then operate the secondstatic switch 420 and the inverter 430 as described above to graduallywalk-in the engine-generator set 14 (block 540). During this process,the inverter 430 may be used to maintain the load and provide reactivepower and/or harmonic compensation as described above. Once theengine-generator set 14 is able to carry the load 20, the inverter 430may be deactivated (blocks 550, 560). In some embodiments, the inverter430 may remain active and continue to provide reactive power, harmoniccompensation and/or other power conditioning while the engine-generatorset 14 serves the load. The inverter 430 may also be operated tofacilitate power flow from the engine-generator set 14 to the DC source440 for battery charging.

FIG. 6 illustrates an exemplary implementation of a three-phase UPSsystem 600 according to some embodiments of the inventive subjectmatter. The system 600 has an AC input 601 configured to be coupled toan engine-generator set 14 (e.g., directly or via one or moreintermediate components, such as a transfer switch) and an AC outputconfigured to be coupled to a load 20. The system 600 includes aninverter circuit 630 including a plurality of half-bridge circuits 632coupled to a DC/DC converter 642 that generates a DC voltage vDC from abattery voltage vB generated by a battery 644. Respective inductors 634couple the half-bridge circuits 632 to the AC output 602. The system 600further includes a plurality of SCR-based static switches 622 coupled tothe AC input 601. Respective inductors 624 couple the static switches622 to the AC output 602. A control circuit 650 is configured to controlthe static switches 622 and the half-bridge circuits 632 responsive tovarious inputs, such as an AC voltage vAC produced by theengine-generator set 14, a DC voltage vB produced by the battery, a DCvoltage vDC produced by the DC/DC converter, a load current il, a loadvoltage vl, a bypass current ibyp through the static switches 622, andan inverter current iinv at the output of the inverter 630. The controlcircuit 650 may also operate response to other information, such asinformation pertaining to the engine-generator set 14. Such informationmay include, for example, information relating to the generator's powerdelivery capability, stability, and the like. The control circuit mayalso operate responsive to information pertaining to external switches(e.g., transfer switches) that couple the AC input 601 to various powersources.

According to some embodiments, the control circuit 650 may operate thestatic switches 622 and the half-bridge circuits 632 to walk-in theengine-generator set 14 while maintaining a desired voltage and currentat AC output 602. For example, the control circuit 650 may graduallyincrease a conduction (“on”) interval tc of the static switches 622,while using the inverter 630 to maintain the desired current and voltageat the AC output 602.

The walk-in may occur in any of a number of different ways. For example,as illustrated in FIG. 7, the conduction interval tc of the SCRs of thestatic switches 622 may be increased in discrete steps over a series ofdiscrete time intervals by changing the firing angles of the SCRs. Thevariance of the conduction interval tc may also be limited subject toindications of desirable generator performance.

Referring to FIG. 7, the conduction interval tc of the static switches622 may be incrementally increased (block 710) by incrementally changingthe firing angles of SCRs of the static switches 622, and the inverter630 responsively adjusted to maintained the load (block 720). After apredetermined time interval has elapsed without indication of anundesirable generator state, the conduction interval tc may again beincrementally increased (blocks 730, 710). An undesirable generatorstate may be indicated by, for example, waveform degradation and/orsignaling from the control system of the engine-generator set 14 (e.g.,the engine-generator set 14 may generate a signal in response to anunacceptably high error in its speed control). If an undesirablegenerator state is detected, the static switch conduction interval tcmay be reduced (blocks 730, 750). If the predetermined time interval hasnot yet elapsed and the generator state is acceptable, the static switchconduction interval tc may be maintained until the predetermined timeinterval has elapsed (blocks 730, 740).

The static switch conduction interval tc may also be controlledresponsive to measure of a performance margin of the engine-generatorset 14. For example, a control circuit of the engine-generator set 14may be configured to signal a degree of available performance margin,e.g., a signal derived from an error signal of a speed control loop ofthe engine-generator set 14, and the control circuit 650 may control thestatic switch conduction interval tc responsive to this measure. Thismay allow for faster walk-in of the engine-generator set 14 incomparison to the process illustrated in FIG. 7.

Referring to FIG. 8, the static switch conduction interval is increasedconcurrent with adjusting inverter operation to maintain the load(blocks 810, 820). As long as sufficient margin for generator operationis indicated, the conduction interval is increased and inverterresponsively adjusted (blocks 830, 810). If an insufficient generatormargin is indicated, however, the conduction interval may be maintainedor reduced as indicated by the generator margin (blocks 830, 840). Itwill be appreciated that the static switch conduction interval and theinverter operation may be continuously adjusted using, for example,proportional feedback control loops.

According to various embodiments, a static switch and inverter may beused in a number of different ways to provide walk-in as describedabove. For example, in some embodiments, a UPS system may alternate aload between the generator and the inverter during non-overlapping timeintervals. In further embodiments, the inverter may remain active whilethe generator is coupled to the load, but may be used in a mode in whichit acts as a current source to provide real and reactive power and/orcompensate for harmonics.

Referring to FIG. 9, in response the loss of a primary (e.g., utility)source, the inverter is used to provide power to the load (block 905).When the system determines that transfer to a generator is appropriate(e.g., after the generator has been started and brought up to a nominaloperating condition), the inverter is synchronized to the generator'soutput and a value x, corresponding to an amount of a third of a cycle(for a three-phase system) of the generator AC waveform for which theload is to be coupled to the generator by a static switch, isinitialized (blocks 910, 915, 920). The inverter is turned off at pointduring the generator voltage waveform corresponding to a third of acycle minus the value x (block 925). The inverter may, for example, beplaced in a standby mode in which its power switching devices aredisabled (e.g., placed in a high impedance state), while the inverter'scontrol circuitry remains active and continues to be synchronized to thegenerator waveform. After a delay suitable to ensure the inverter hascommutated off, the static switch is triggered to couple the generatorto the load (block 930). The value x is incremented and, if still lessthan a third of a cycle of the generator waveform, the static switch isturned off (e.g., allowed to commutate off at the next zero crossing ofthe generator voltage) (blocks 935, 950). The inverter is then turned onat the state of a new third of a cycle and provides power to the loadfor a reduced period of time (blocks 955, 925). If the value x equals orexceeds the length of a third of a cycle, the static switch is kept onto allow the generator to continue to power the load (blocks 940, 945).The inverter may remain deactivated at this point or may be selectivelyused for power conditioning (e.g., harmonic suppression) or otherpurposes. The amount Δx by which the value x is incremented may be afixed or otherwise predetermined value and/or may be a value that isadjusted based on monitored system parameters (e.g., voltage, frequency,generator speed loop error, etc.).

In further embodiments, when walk-in of the generator commences, aninverter may be switched from a mode in which it solely controls thevoltage/frequency at the load to a parallel operation mode. Referring toFIG. 10, in response the loss of a primary (e.g., utility) source, theinverter is used to provide power to the load (block 1010). When thesystem determines that transfer to a generator is appropriate (e.g.,after the generator has been started and brought up to a nominaloperating condition), the inverter is transitioned to a paralleledoperation mode in which the inverter may provide real or reactive powerto the load to reduce loading of the generator and/or to provideharmonic compensation to produce a desirable waveform at the load, e.g.,limit frequency deviation and/or voltage droop (blocks 1020, 1030).While the inverter is operating in this mode, the static switch ismodulated to gradually increase the “on” time in which the generator iscoupled to the load. The on time of the static switch may be increasedin a predetermined manner and/or may be adjusted based on monitoredsystem parameters.

It will be appreciated that the system and operations illustrated inFIGS. 6-10 are provided for purposes of illustration, and thatembodiments of the inventive subject matter are not limited thereto. Forexample, although the system of FIG. 6 uses an SCR-based static switch,other types of switches may be used and controlled in the same ordifferent manner. For example, a transistor-based switch may becontrolled using pulse-width modulation or other techniques, rather thanthe firing angle control of waveform-commutated SCRs described above.Such transistor-based switching devices may also be used for powerfactor control and other functions. It will be further appreciated thatthe inventive subject matter may be power distribution system such asthe configurations shown in FIGS. 2 and 4, as well in other powerdistribution system arrangements.

In the drawings and specification, there have been disclosed exemplaryembodiments of the inventive subject matter. Although specific terms areemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the inventive subject matterbeing defined by the following claims.

That which is claimed:
 1. An apparatus comprising: an AC output; aninverter coupled to the AC output and configured to provide power at theAC output; a switch configured to selectively couple an AC source to theAC output; and a control circuit configured to repeatedly turn theswitch on and off and to repeatedly activate and deactivate the invertersuch that the switch is on and the inverter is active in respectiveseparate intervals during a transfer of a load at the AC output betweenthe inverter and the AC source.
 2. The apparatus of claim 1, wherein theswitch comprises a static switch.
 3. The apparatus of claim 2, whereinthe control circuit is configured to repeatedly turn on the staticswitch in spaced apart conduction intervals and to control durations ofthe conduction intervals of the static switch to gradually increasepower flow from the AC source to the AC output.
 4. The apparatus ofclaim 3, wherein the control circuit is configured to change theconduction interval durations in discrete steps over respective discretetime intervals to control the power flow from the AC source to the ACoutput.
 5. The apparatus of claim 3, wherein the AC source comprises agenerator and wherein the control circuit is configured to vary theconduction interval durations responsive to a signal indicating aperformance margin of the generator.
 6. The apparatus of claim 1,wherein the control circuit is configured to operate the inverter toprovide reactive power compensation when the AC source is providingpower to the load.
 7. The apparatus of claim 1, wherein the controlcircuit is configured to operate the inverter to provide harmoniccompensation when the AC source is providing power to the load.
 8. Theapparatus of claim 1, wherein the switch comprises a silicon controlledrectifier (SCR) based switch.
 9. The apparatus of claim 1, wherein theswitch is a transistor-based switch.
 10. The apparatus of claim 1,wherein the inverter is constrained to be inactive during the intervalsin which the switch is on.
 11. A method comprising: providing power to aload from an inverter; and repeatedly turning a switch that couples anAC source to the load on and off and repeatedly activating anddeactivating the inverter such that the switch is on and the inverter isactive in respective separate intervals during a transfer of the loadbetween the inverter and the AC source.
 12. The method of claim 11,wherein repeatedly turning the switch that couples an AC source to theload on and off and repeatedly activating and deactivating the invertersuch that the switch is on and the inverter is active in respectiveseparate intervals during a transfer of the load between the inverterand the AC source comprises repeatedly turning on the switch in spacedapart conduction intervals and controlling durations of the conductionintervals to gradually increase power flow from the AC source to theload.
 13. The method of claim 12, further comprising changing theconduction interval durations in discrete steps over respective discretetime intervals to control the power flow from the AC source to the load.14. The method of claim 12, wherein the AC source comprises a generatorand wherein the method further comprise varying the conduction intervaldurations responsive to a signal indicating a performance margin of thegenerator.
 15. The method of claim 11, further comprising operating theinverter to provide reactive power compensation when the AC source isproviding power to the load.
 16. The method of claim 11, furthercomprising operating the inverter to provide harmonic compensation whenthe AC source is providing power to the load.
 17. The method of claim11, wherein the inverter is constrained to be inactive during theintervals in which the switch is on.
 18. An apparatus comprising: an ACoutput; an inverter coupled to the AC output and configured to providepower to a load at the AC output; a transistor-based switch configuredto selectively couple an AC source to the AC output; and a controlcircuit configured to repeatedly turn the transistor-based switch on andoff to provide spaced apart intervals in which the switch conducts toincrease power flow to the load from the AC source during a transfer ofthe load from the inverter to the AC source and to cause the inverter toprovide power to the load during the transfer.
 19. The apparatus ofclaim 18, wherein the control circuit is configured to alternately turnon the switch and activate the inverter in respective separate intervalsto transfer a load at the AC output between the inverter and the ACsource.